Technology giant Apple has been accused by a US Senate committee of tax evasion for allegedly exploiting the differences between American and Irish tax laws.

According to the Senate Permanent Subcommittee on Investigations, Apple has not acted illegally but by using an unusual tax structure has managed to keep billions of dollars of profits tax-free in Irish subsidiaries. The report comes a day before Apple’s CEO is due to testify before Congress on the matter.

The main subsidiary, which includes Apple’s retail stores throughout Europe, has not paid any tax in the last five years. The Senate report shows that the Cork-based subsidiary received $29.9 billion in dividends from 2009-2012, which equates to 30 percent of Apple’s total worldwide net profits.

Apple has denied the claims saying it does not use “tax gimmicks“ and indicated it paid nearly $6 billion in taxes to the US Treasury in 2012.

But committee member John McCain told reporters: “Apple claims to be the largest US corporate taxpayer, but by sheer size and scale, it is also among America’s largest tax avoiders.”

The Senate group is examining the methods used by large corporations to move profits offshore. Many large firms avoid repatriating their foreign earnings to escape the top 35% tax rate.

Ireland has said it is not to blame for Apple’s low global tax payments. Deputy Prime Minister Eamon Gilmore told Irish station RTE, “They are not issues that arise from the Irish taxation system…They are issues that arise from the taxation systems in other jurisdictions”.